1. Field of the Invention
The present invention relates to an ophthalmic apparatus for measuring refractive characteristic of an eye to be measured and in particular, to an ophthalmic apparatus for measuring eye refractive power in a plurality of meridional directions of the eye to be measured and cornea shape based on curvature distribution in a cornea of the eye.
2. Description of Related Art
Conventionally, as an ophthalmic apparatus for measuring refractive power of an eye to be measured, it is well-known an apparatus through which refractive power is measured by projecting a measuring target onto a fundus of the eye to be measured and detecting an image of the target projected onto the fundus. Such kind of the apparatus has a purpose to prescribe glass lens or contactlens, therefore the apparatus is comprised so as to be able to determine three values such as the spherical refractive power, the cylindrical refractive power and the cylindrical axial angle, these values being necessary for correctly preparing the glass lens or the contactlens. That is to say, in the detecting optical system of the above apparatus, it is adopted, for instance, a method in which two measuring luminous fluxes are projected to positions symmetrical about the visual axis of the measured eye or a method in which one luminous flux is projected to the center of the pupil in the eye and the reflected luminous flux from the fundus is introduced through three openings which are arranged at positions conjugated with the cornea of the eye, thereby diffusion of the luminous flux to three meridional direction is detected.
The refractive power of the spherical surface, the cylindrical surface and the astigmatism axial angle measured through the above apparatus are obtained by treating the measured data in the meridional directions through the equation sin 2 .theta..
Even in the conventional apparatus, it can obtain the necessary and sufficient results so long as the apparatus has the purpose for prescribing glass lens or contactlens. Here, the measurement of the eye is conducted in the apparatus based on the assumption that an eyeball is symmetrical. However, as a matter of fact, it is generally said that more than 1/3 of men or women who have refractive ametropy, have asymmetrical astigmatism. Thus, in the conventional apparatus, since the mean refractive power of the eye is measured in neglecting the above point, it is not enough at a point that the correct distribution of the refractive power in the eye cannot be measured.
Further, it is recently known an ophthalmic treatment in which the surface of the cornea is ablated or incised by excimer laser, thereby the curvature of the cornea is changed and refractive ametropy can be corrected. In the ophthalmic treatment, the operation has to be conducted based on the correct data of the refractive power of the eye to be operated and it is necessary the detailed data thereof to observe the status of the operated eye after the operation. However, in the above conventional apparatus, it cannot be obtained sufficient data of the refractive power of the eye which is necessary for the above operation.
And in case that the above ophthalmic treatment is conducted, at first, it is necessary to know both the detailed data of the cornea shape and the refractive power value of the eye to be operated.
Here, as the apparatus through which the cornea shape data can be obtained in detail, it is conventionally known an apparatus utilizing so-called Cornea Topography through which the curvature distribution of the cornea can be measured over a wide range thereof and the measured distribution can be visualized. In such the apparatus, the curvature distribution of the cornea is visualized as follows. That is, a reflected image of the cornea according to Placido-Ring projected onto the eye to be measured is photographed by a CCD camera and a Placido-Ring image is detected by conducting picture processing. Further, the curvature distribution over substantially entire range of the cornea of the eye is obtained by conducting calculation processing in which positional data of the Placido-Ring image located at a predetermined position is processed through a well-known manner and thereafter the curvature distribution is graphically visualized on a T.V monitor. In the apparatus that graphic display is done, it is also known an apparatus in which graphic display on T.V. monitor is conducted through color graphic so that the curvature distribution of the cornea can be easily understood.
And when the ophthalmic treatment mentioned above is conducted, it has to be used two kinds of the apparatuses, one being the apparatus utilizing Cornea Topography for measuring the cornea shape and the other being the apparatus for measuring the refractive power value, in order to obtain both the cornea shape data and the refractive power value of the eye to be operated before the operation.
However, if the two apparatuses are necessary to prepare, there is a problem that not only equipment investment is increased but also wide space for equipping the apparatuses has to be secured. And there is also a problem that an inspector has to move between the apparatuses while measuring. Further, data processing of the data obtained from the two apparatuses is very troublesome, thus the measurement efficiency becomes worse.